The present invention relates to heat pumps, and more particularly to a heat exchanger and a heat pump circuit for a direct expansion heat pump.
Heat pumps have long been used as year-round air conditioning systems that operate in a heating cycle and a cooling cycle. Heat pumps are generally more efficient than conventional heating and cooling systems because they transfer rather than create heat. The fundamental principles of heat pump operation are simple. In the heating cycle, the heat pump draws heat from an outside heat source such as earth, air, or water and transfers it to the conditioned space. In the cooling cycle, the heat pump abstracts heat from the conditioned space and dissipates it into an outside heat sink.
In a conventional heat pump circuit, refrigerant is pumped through an outdoor coil where ambient air either heats or cools the refrigerant. The heated or cooled refrigerant is pumped through an indoor coil to heat or cool the conditioned space. Experience has revealed that this type of heat pump is relatively inefficient, largely because ambient air does not function as a stable heat source/sink. A number of "heat exchangers" have been developed to increase the efficiency of heat pumps by utilizing the earth or outside water as the heat source/sink. Heat exchangers replace the conventional outdoor coil and can be buried in the ground or submerged in a well, lake or river to facilitate heat transfer between the refrigerant and the heat source/sink.
Heat exchangers are available in a variety of designs. Among the most popular designs are "U" shaped and coaxial heat exchangers. A typical "U" shaped design includes a liquid line and a vapor line that are connected to form opposite legs of a "U". In a conventional coaxial design, the liquid line extends coaxially into the center of a heat transfer tube. The end of the liquid line is open to allow refrigerant to flow between the line and tube. In the heating cycle, liquid refrigerant flows into the liquid line where it receives heat from the heat source. The refrigerant evaporates and flows out of the heat exchanger through the vapor line or heat transfer tube. The vaporized refrigerant flows through an indoor coil where it condenses. The heat released in the coil during the phase change is passed into the conditioned space. The liquified refrigerant then flows back into the heat exchanger to repeat the cycle. In the cooling cycle, vaporized refrigerant enters the vapor line or heat transfer tube where it condenses to transfer heat to the heat sink. The liquid refrigerant passes through the liquid line into an indoor coil where the liquid refrigerant evaporates by abstracting heat from the conditioned space. The vaporized refrigerant then flows back into the heat exchanger to repeat the cycle.
It is well known that there is a refrigerant imbalance between the cooling and heating cycles. During the cooling cycle, liquid refrigerant must fill the entire liquid line before it returns to the circuit for use. Consequently, a tremendous amount of liquid refrigerant is needed during the cooling cycle. However, the heating cycle does not require such a large volume of refrigerant because the vaporized refrigerant expands quickly and rises, returning to the circuit for use. To overcome the imbalance, some manufacturers provide the system with a refrigerant receiver that stores the refrigerant during the heating cycle when it is not needed. Refrigerant receivers increase the size and cost of the system. Alternatively, some systems include multiple heat exchangers some of which are shut down during the cooling cycle. The refrigerant passing through the shut-down exchangers during the heating cycle is thereby made available for use by the remaining exchangers during the cooling cycle. This type of system is relatively expensive to manufacture and install. And finally, some systems include a control system that drains refrigerant from the system during heating and return it to the system during cooling. Again, the control system increases the size and cost of the heat pump system.
During the cooling cycle, a significant amount of heat dissipates from the vapor line as the refrigerant condenses. Some of this heat is transferred to the liquid line where it heats the liquid refrigerant causing it to vaporize or "flash off". This reduces the efficiency of the system. To overcome this problem, a variety of methods for thermally insulating the liquid line from the vapor line have been developed. One simple method is to increase the distance between the vapor line and the liquid line. A second method is to wrap the liquid line with insulation. A third method is to separate the vapor line and liquid line by a vacuum. All of these methods increase the manufacturing and installation costs of the system.
In addition, in northern climates there is disparity in the amount of heat exchange area needed during the heating cycle and the cooling cycle. Some manufacturers have addressed this problem by providing the system with multiple heat exchangers. During the cooling cycle, some of the heat exchangers are shut-down to provide the appropriate heat exchange area.